There is a large demand for alkylbenzenes and synthetic lubricants (synlubes). Alkylbenzenes are often used as detergents in a variety of applications, for example, as lubricant oils. The preferred alkylbenzenes have linear (as opposed to branched) alkyl groups, and are referred to as linear alkylbenzenes. Linear alkylbenzenes are preferred over branched alkylbenzenes because of their relatively high rate of biodegradability.
Currently, these products are prepared by alkylating normal alpha olefins (NAOs), or isomerized olefins derived from normal alpha olefins, with benzene or toluene. The NAOs are typically made from ethylene, which is a relatively expensive raw material. Accordingly, the cost of the alkylbenzenes and synlubes is relatively high. NAOs can also be prepared by wax cracking and by modified ethylene oligomerization processes.
Linear alkylbenzenes can also be prepared from high purity unbranched paraffins by dehydrogenating the paraffins to form olefins, and then alkylating aromatic rings with the olefins. However, this approach is limited by the relatively high cost of the paraffinic starting material and the limited supply of high quality unbranched paraffins. If the paraffins are not extremely pure, but rather, include isoparaffins or naphthenes, the catalysts tend to foul and the products tend not to perform adequately.
One type of synthetic lubricant is derived from 1-decene that has been trimerized and hydrogenated to form a T-shaped C30 isoparaffin. The particular shape of the molecule provides it with a relatively high viscosity index and low pour point, which is desirable for synthetic lubricants. However, the decene trimer is not unique. Many alkylcyclohexanes also have relatively high viscosity indexes (VI""s) and low pour points (Briant et al., xe2x80x9cRheological properties of lubricantsxe2x80x9d, Editions Tecnip (Chapter 7 (1989).
It would be desirable to provide additional methods for forming alkylbenzenes and alkylcyclohexanes. The present invention provides such methods.
In its broadest aspect, the present invention is directed to an integrated process for preparing alkylbenzenes, sulfonated alkylbenzenes and/or alkylcyclohexanes from syngas. In the first step, syngas is reacted under Fischer-Tropsch conditions to form one or more product streams that include C6-8 and C18-26 fractions. These fractions can be isolated, for example, via conventional fractional distillation. Both fractions are optionally but preferably treated, for example, by hydrotreating or extraction, to remove oxygenates and other by-products of the Fischer-Tropsch synthesis.
The C6-8 fraction can be converted to aromatics via catalytic reforming chemistry, preferably using the AROMAX(copyright) Process. The C6-8 fraction is ideal for use in the AROMAX(copyright) Process, because it tends to have low levels of sulfur, a known poison for the catalyst used in the AROMAX(copyright) Process.
The C18-26 fraction tends to be highly linear, and also has low levels of impurities known to adversely affect processes for alkylating aromatics with olefins. Depending on the particular Fischer-Tropsch conditions, the fraction may include sufficient olefins and alcohols such that it can be directly reacted with aromatics to form alkylbenzenes. The aromatics and C18-26 hydrocarbons used to form the alkylbenzenes can be derived exclusively from the C6-8 and C18-26 fractions from the Fischer-Tropsch reaction, or can optionally be combined with aromatics and/or C18-26 hydrocarbons from other feedstocks, assuming that the other feedstocks do not include impurities that would have a detrimental effect on the subsequent chemistry.
The paraffinic portion of the C18-26 fraction can be dehydrogenated to form olefins and reacted with the aromatics. The alkylbenzenes can be used, for example, as lubricant oils, or can be sulfonated to form detergents.
The alkylbenzenes can be reduced to form alkylcyclohexanes. In one embodiment, the hydrogen produced during the catalytic reforming chemistry can be used to hydrogenate the alkylbenzenes to form the alkylcyclohexanes. The resulting alkylcyclohexanes can be used, for example, as synlubes or as a component in synlube compositions. Preferably, the lubricant compositions, including the alkylbenzenes and/or alkylcyclohexanes, also include conventional lubricant additives.
In one embodiment, the Fischer-Tropsch chemistry is performed in two separate reactors, in order to maximize the relative amounts of C6-8 and C18-26 fractions. The first reactor can be set up using conditions in which chain growth probabilities are relatively low to moderate, and the product of the reaction includes a relatively high proportion of low molecular (C2-8) weight olefins and a relatively low proportion of high molecular weight (C30+) waxes. This set of conditions optimizes yields of the C6-8 fraction used to form the aromatic rings which are to be alkylated with the C18-26 paraffins. Preferred catalysts are iron-containing catalysts.
The second reactor can be set up using conditions in which chain growth probabilities are relatively high, and the product of the reaction includes a relatively low proportion of low molecular (C2-8) weight olefins and a relatively high proportion of high molecular weight (C30+) waxes. Preferred catalysts are cobalt-containing catalysts. This set of conditions optimizes yields of the C18-26 fraction used to alkylate the aromatic rings.